He said everything should be done “to destroy and eliminate that threat”.

At least 800 Britons have gone to Syria and Iraq to fight for IS and 130 of those have been killed in conflict.

Mr Williamson, who took over as defence secretary last month, told the newspaper: “I do not believe that any terrorist, whether they come from this country or any other, should ever be allowed back into this country.”

British fighters who had fled to other countries would also be found and stopped from returning to the UK, he said, adding that there would be no “safe space” abroad for them either.

“We have got to make sure that as (they) splinter and as they disperse across Iraq and Syria and other areas, we continue to hunt them down,” he said.

Mr Williamson’s predecessor Sir Michael Fallon said in October that British IS fighters in Syria and Iraq had made themselves “a legitimate target” who could end up on “the wrong end of an RAF or USAF missile”.

His comments came after it was reported that British IS recruiter Sally-Anne Jones had been killed in a US drone strike in Syria in June.

And Rory Stewart, the minister for international development, said the “only way” to deal with British IS fighters in Syria is “in almost every case” to kill them.

He said they can expect to be killed because of the “serious danger” they pose to the UK’s security.

In contrast, Max Hill QC, the Independent Reviewer of Terrorism Legislation, has said Britons who joined IS through “naivety” should be spared prosecution and instead be reintegrated into society if they return home.